In non-volatile memories in which each memory cell has only one associated bit line, such as ROMs, EPROMs, and EEPROMs single-ended sense amplifiers have been frequently used. Techniques using dummy cells have been developed so that double-ended sense amplifiers which detected a voltage differential between the dummy cell and a selected cell were used. When only a distinction between two states was to be detected a relatively large noise margin between the two states was developed. Consequently, the relatively large differential between states was easily detected.
In a four state ROM, however, where there are more than two states in a given voltage range, precision detection is required. This is particularly true in a virtual ground system in which two adjoining ROM cells share a source connection which is selectively switched to ground potential when one or both of the two ROM cells are selected. Virtual ground systems have been used to reduce chip size. A characteristic of virtual ground system, however, is a limitation on the voltage swing on the bit line. A bit line precharged to 4.0 volts, for example, may be allowed to drop to no lower than 3.0 volts in a virtual ground system. The lower allowed limit is affected by the threshold voltage of the transistors used and by word line voltage. Consequently, the voltage swing between the most conductive and least conductive states can easily be only 1.0 volt. Consequently, differentiating between two states which are 1.0 volt apart is substantially easier than differentiating four states over the same range.
Another typical problem in ROMs is known as power supply voltage slew. A typical ROM simply has a load which establishes a bit line voltage or has a clocked transistor coupled between a power supply and a bit line for precharging bit lines. This can cause the bit lines to be charged to too high of a voltage if the power supply incurs a positive pulse between accesses. Bit lines may remain charged to a voltage higher than that of the bit line or lines which have dummy cells.